The field of wireless technology is currently undergoing a revolution, and is experiencing exponential growth. Mobile phones, once considered a novelty and referred to as xe2x80x9ccar phonesxe2x80x9d are now ubiquitous, and cordless phones in the home are commonplace. A new batch of wireless personal digital assistants, phones, and Bluetooth enabled computer peripherals are now entering the market, with wireless Internet access as a driving force. A differential to single-ended converter for use in wireless transmitters as a driver for power amplifiers is described. This converter has a large output swing, high efficiency, and good linearity.
Wireless devices typically transmit and receive data through the air on high frequency electromagnetic waveforms, though some systems, such as satellite dishes and pagers simply receive, and others merely transmit. Data transmission is begun by encoding the data to be transmitted. This encoded data typically has a data rate of 100 kHz to 100 MHz and is used to modulate a high frequency carrier signal. The carrier signal is often in the 800 MHz to 10 GHz range. The modulated carrier signal is then applied to an antenna for broadcasting. The broadcast signal is referred to as a radio frequency (RF) signal. Reception involves receiving the RF signal on an antenna, and filtering undesired spectral components. The signal is demodulated, filtered again, and decoded.
FIG. 1A is a simplified block diagram of a transmitter portion 100 used in these wireless applications. Included are a mixer 120, power amplifier driver 130, power amplifier (PA) 140, antenna 150, and voltage controlled oscillator (VCO) 160. VCO 160 generates a local oscillator (LO) signal, the frequency of which is referred to as the carrier frequency. This LO signal is applied to mixer 120 which multiples it with the Baseband data signal on line 110. The output of mixer 120 is a differential output. The power amplifier driver 130 provides gain and converts this differential signal to a single-ended signal useful for driving the PA 140. The PA 140 is a single-ended input and output circuit designed to provide output power for driving the antenna 150.
FIG. 1B is a simplified schematic of a conventional differential to single-ended converter, useful as a power amplifier driver. Differential pair M1 170 and M2 180 are driven by an input signal Vin across input terminals 175 and 185. A time varying (AC) signal at Vin modulates the gate-to-source (VGS) voltages of M1 170 and M2 180, thus generating AC currents in their drains. The drain current of M2 is applied across the load R1 190, which generates an output voltage Vout on line 195. This architecture is popular since it is simple and has good bandwidth. But one drawback of these circuits is that half of the potential gain is lost. In this specific example, it is because the current in the drain of M1 170 is shunted to VCC, and does not contribute to the Vout signal. This means that half of the bias current supplied by current source 195 is wasted. Therefore, there is a need to provide a differential to single-ended converter which makes more efficient use of its supply current.
Three key components of a wireless transmitter are the mixer, the power amplifier, and the converter which is connected between them. The mixer and converter are typically on an integrated circuit with other portions of the receive and transmit channels. Often the power amplifier is off-chip, such that a higher bandwidth process, gallium arsenide is an example, may be used. The mixer is usually differential in nature, and is often a Gilbert multiplier, such that a differential current output is available. The power amplifier is typically single-ended voltage in and single-ended voltage out, with the output connected to the antenna. Therefore, the converter needs to convert the differential current signal from the mixer to a single-ended voltage signal for the power amplifier. Also, the converter may need to provide an output signal capable of driving a signal off-chip through the extra capacitance of bond wires, printed circuit board traces, package leads, and the like. Therefore, the converter needs to be a high bandwidth differential current to single-ended voltage output circuit. Transmitter performance is enhanced if the converter has a large output swing and voltage gain.
Accordingly, embodiments of the present invention provide methods and apparatus for converting differential signals to single-ended signals. One embodiment takes advantage of inductive loads to increase the output swing. Various embodiments use loads, such as resistors and inductors, to convert a first side of a differential current to a voltage. The voltage can then be converter by a transconductor, such as an NMOS or PMOS device, to a current that may be then added to a second side of the differential current. The added currents are then applied across another load, such as a resistor or inductor, to generate an output voltage.
One exemplary method includes receiving a differential signal comprising a first current and a second current and applying the second current to a first load to generate a first voltage. A third current is generated in response to the first voltage. The first current is summed with the third current and applied to a second load to generate the single-ended signal.
A further exemplary embodiment of the present invention provides a transmitter, where the transmitter has a converter circuit with a first terminal and a second terminal. In this converter circuit, the first terminal receives a first current and provides an output voltage, and the second terminal receives a second current. The converter circuit includes a first load, coupled between a third terminal and the second terminal, which is configured to convert the second current to a first voltage. A transconductance is coupled to the second terminal and the first terminal, and is configured to convert the first voltage to a third current. In one embodiment, the third current is phase shifted or inverted relative to and proportional to the first voltage. A second load is coupled between the a fourth terminal and the first terminal and is configured to convert the sum of the first current and the third current to the output voltage.
Yet another embodiment of the present invention provides a converter which includes a first inductor coupled between a first terminal, which receives a first current signal, and a second terminal. A second inductor is coupled between a third terminal, which receives a second current signal, and a fourth terminal. A first transistor is coupled between the third terminal and a fifth terminal, and it has a control electrode coupled to the first terminal. The first current input and the second current input form a differential current signal, and a single-ended voltage output is generated at the third terminal in response to the differential current signal.
A better understanding of the nature and advantages of the present invention may be gained with reference to the following detailed description and the accompanying drawings.